Ask them to create a new Sprint account for you, and run any necessary credit checks.

Note that Sprint requires all phones on the free plan remain active for the first 4 months.

Choose either:

Buy a new phone before activating the free plan:
Before activating this deal, buy a new phone from anywhere that allows a full-price buy without activation.
So, not a Sprint store.
Not a Costco.
Valid buys include an iPhone bought from an Apple store, a Windows phone bought from a Microsoft Store, a full-price buy from a generic/local phone retailer, or bought directly from Sprint customer service.
Note that an iPhone bought from an Apple Store must be activated with a SIM card during purchase, so use your old phone's SIM card (do not use a Sprint SIM card yet).
Or,

Use your existing phone:
Must work for the next 4 months.

Get a Sprint SIM card for your phone(s).
This can be done by doing one of these:

When talking to the Sprint sales rep by calling 866-782-8777, they can mail cards to you for $10 to $15, or

Go to an Apple store:
Many Apple stores give out Sprint SIM cards for free just by asking, or

Go to a Sprint store:
Ask for Sprint SIM cards from your local Sprint store.
They may charge for them.
Be sure they do not activate the SIM card.

Transfer your existing phone number(s) and phone(s) to Sprint by calling 866-782-8777.
This will activate the plan.
Remember, each phone number must be transferred from another major wireless provider in the US (AT&T, Verizon, T-Mobile, etc).
You cannot activate brand new phone numbers with this deal.

Mark your calendar for May, 2019.
That is when Sprint will switch your plan to full price ($60/mo for the first line, $40/mo for the second, and $30/mo each additional line).
Re-negotiate or change plans then by calling Sprint at that time.

For the next year, your actual monthly bill will be free plus government taxes, which can come to about $3 or $5 per month per line.
Our plan of 2-lines costs us about $6 per month after taxes.
This is a huge savings over Verizon's price of $130/mo (plus taxes) for 2 lines.
After being on this plan for 4 months, you can purchase or lease a new phone and remain on the 1-year free plan.
Our advice when on the free 1-year Sprint service:

Within the first 4 months, do not cancel any phones on your account.
If you switch phones or cancel service within 4 months, you will lose the remainder of your 1-year free service.

Do not apply any upgrades to your account, as it may ruin your free status for the year.

Do not buy phones from a Sprint store which could mess up your free plan.

Good luck.
This deal is easy to get, as long as you understand how to get it.